Airborne Laser Scanning - Assignment Example

Contents 1Describe the required flow line to generate a regular-grid digital elevation model (DEM) from Airborne Laser Scanning (ALS) data and comment on the expected accuracies 2 2Least Squares adjustment, as applied in geospatial engineering, is based on two principles – one relating to the computation model and one relating to the observed data. Explain 3 3The establishment of a GNSS CORS Network is fundamental in providing a national framework for spatial data. Detail the requirements for such a network and, using a national example, outline the survey and engineering positioning products from such a network. 5 4Critically discuss the development and future direction of GNSS. Your answer may focus on one specific GNSS system and should give reference to the modernization of satellite vehicles and modes of operation for geospatial data provision 6 References 8 1 Describe the required flow line to generate a regular-grid digital elevation model (DEM) from Airborne Laser Scanning (ALS) data and comment on the expected accuracies Airborne Laser Scanning is a remote sensing technology. It actively conveys light pulses towards the targeted object, hence receiving the light that is scattered and reflected by these objects (Reutebuch, Andersen & McGaughey, 2005, p.289). An Airborne Laser Scanning system is composed of an Inertial Measurement Unit, a Laser Scanner Unit, and a Global Positioning System receiver. The first crucial step in DEM generation from Airborne Laser Scanning is separating the Airborne Laser Scanning points into the terrain or ground, as well as non-terrain or non-ground points. First, a rough estimate of the land surface is calculated using equivalent weights for all the points (Reutebuch, Andersen & McGaughey, 2005, p.291). The estimated surface forms the average surface between the non-terrain and the terrain points. The oriented distance between the surface and the points is then calculated and is called the residual. Normally, ground points have negative residuals whereas non-terrain points have positive residuals. Every point is then given a weight as per its residual. The points having negative residuals are given high weights that are taken to be the terrain surface, whereas those points having small weights are considered to be non-terrain points (Reutebuch, Andersen & McGaughey, 2005, p.292).This process is repeated to ensure that the surface gets much closer to the ground. The second step is data reduction. It is aimed at achieving an optimum balance between volume of data and density of sampling (Robinson, 1994, p.807). This helps in optimizing the cost of data collection. In optimal interpolation, incredibly detailed DEMS with high resolution and accuracy are obtained from high density Airborne Laser Scanning data. However, since there is no scope for matching the acquisition frequency of data by terrain type during Airborne Laser Scanning data collection process, some oversampling is typically unavoidable (Robinson, 1994, p.808). Consequently, the processing times and the data storage requirements become higher than usual. This calls for the strategies for dealing with greater volumes of terrain data to ensure the accuracy; supposing efficiency is to be realized. Data reduction can result in an operationally sized and a more manageable terrain set of data for DEM generation. Breakline extraction is another critical step. Breaklines are skeleton or structure lines such as valley lines or ridge lines. These are essential features of the terrain since they describe changes in the surface. They give elevation information and also represent terrain information concerning their surroundings (Robinson, 1994, p.809). Breaklines provide more significant information about terrain compared to other points. Preservation, as well as integration of breaklines in the making of DEM, greatly contributes to attaining a hydrologically enhanced, reliable, and morphologically correct DEM (Robinson, 1994, p.811). Their integration in the making of DEM ensures drainage structure and good shape in the calculated DEMS. This is made possible by formulating a drainage implementation condition on the tailored grid values directly from the original streamline data. 2 Least Squares adjustment, as applied in geospatial engineering, is based on two principles – one relating to the computation model and one relating to the observed data. Explain Least squares adjustment is a method of a standard approach to approximating the solution of over-determined systems; that is, sets of equations that have more equations than unknowns (Charles, 2011, p.54). Notably, Least squares imply that the overall solution reduces the sum of the squares of the faults made in the outcomes of every single equation. Least squares adjustment makes use of both conventional observation as well as computation model (Charles, 2011, p.55). In the standard representation model, the solution of an over-determined system is extensively used in the field of geomatics, in the discipline of surveying, photogrammetry, and geodesy. The most significant part of the observation model is data fitting. In the sense of least squares, the best fit minimizes the sum of all the squared residuals (Charles, 2011, p.56). Residual is the difference between the value observed and the fitted value given by the model. When the observations originate from an exponential family involving satisfaction of mild conditions, then least-squares estimates as well as maximum-likelihood estimates become identical. The least squares method can also be derived from a technique of moment estimator If the problem contains considerable uncertainties in the x variable or the independent variable, then the least squares method is said to have problems. In such a case, the suitable methodology to be employed is that of fitting errors in variable models (Charles, 2011, p.57). This amounts to computation model. Least squares problems occur in two ways. First, as linear or ordinary least squares and secondly, as non-linear least squares, in accordance with whether the residuals are linear in all unknowns or not. The linear least-squares problem happens in statistical regression analysis and has a closed-form solution. The non-linear problem, on the other hand, has no closed-form solution. It is usually resolved by iterative modification. The system is linearly approximated at each level of iteration. Core calculations are entailed in this process. 3 The establishment of a GNSS CORS Network is fundamental in providing a national framework for spatial data. Detail the requirements for such a network and, using a national example, outline the survey and engineering positioning products from such a network. GNSS CORS Network is important in giving a national structure for spatial data. Its use requires that certain requirements are adhered to. First, all the measurements within the network must be acquired (Schaffrin & Felus, 2008, p.375) Secondly, the ambiguities within this network are then fixed, and the area correction parameters determined. Also, the measurements for a reference station have to be generated. In addition, GNSS CORS Network requires that the measurements for a virtual reference station are estimated by using the approximate position of the rover. Lastly, baselines as well as the rover position have to be determined using the rover measurements. CORS Networks has been employed in Geodesy and Surveying. It has proved itself as a useful tool in space geodesy aimed at measuring crustal motion as well as locating the global reference frame (Schaffrin & Felus, 2008, p.377). Currently, CORS Networks has become a major tool for present geodesy. The GPS geodetic stations have turned to be permanent reference stations for realizing modern geocentric geodetic datum nationally, monitoring the motion and extending density of geodetic control using GPS techniques. It is also critical to note the contribution of CORS Networks of the reference station of the IGS to geodesy together with its significance to the general GNSS community (Teunissen, Odijk & Zhang 2010, p.226). Many GPS receivers are globally distributed function on a continuous basis for more than ten years. They send data to the centers for IGS analysis as well as to other users. The data for satellite tracking has been of great importance in progressive Geodesy and Surveying. Several countries have also developed active primary networks of GPS CORS to supervise the stability and reliability of their datum (Teunissen, Odijk & Zhang 2010, p.227). This is specifically the case for nations located near or on tectonic plate boundaries that create their datum. 4 Critically discuss the development and future direction of GNSS. Your answer may focus on one specific GNSS system and should give reference to the modernization of satellite vehicles and modes of operation for geospatial data provision The GNSS systems have increasingly acquired desirable positional capabilities that are suitable for environmental monitoring. These have made the GNSS become global and all weather (Hofman-Wellenhof, Lichtenegger & Wasle, 2008, p.23). They are, therefore, able to monitor global environmental phenomena such as global warming, unlike before. By being all weather, the GNSS have been useful during rainy and cloudy periods; a development that has still remained stumbling blocks to Earth-orbiting satellites and radar systems. Also, the modern GNSS systems can offer 24-hour coverage (Hofman-Wellenhof, Lichtenegger & Wasle, 2008, p.23). This development allows for both day and night observation thus permitting continuous monitoring of events for satellite vehicles. From the viewpoint of the user, the maintenance of satellite vehicles has become cheaper due to the GNSS system developments. GNSS have become economical since only a few operators are required to operate the receivers and to process data (Hofman-Wellenhof, Lichtenegger & Wasle, 2008, p.24). Therefore, less time is required to carry out a GNSS survey to attain a solution. Additionally, the modern GNSS can utilize a standard global reference frame such as WGS-84 Coordinate System. The future of WGS-84 GNSS System seems to be brighter. The Russian government has embarked on a program aimed at developing the second and the third generation GLONASS-M and GLONASS-K respectively (Hofman-Wellenhof, Lichtenegger & Wasle, 2008, p.24). These will significantly improve features for instance, by allowing simpler integration with other systems like GPS. The receivers are also undergoing substantial improvements to increase their reliability and quality of the tracked signals; the world soon be flooded with various receivers that will help in tracking of the GNSS satellites. References Charles D G 2011, ‘Adjustment Computations: Spatial Data Analysis’, John Wiley & Sons. Hofman-Wellenhof B, Lichtenegger H, Wasle E 2008, ‘GNSS global navigation satellite system: PS, GLONASS; Galileo and more’ Springer, Wien Reutebuch, S E, Andersen, H-E & McGaughey, R J 2005, ‘Light Detection and Ranging (LIDAR): An Emerging Tool For Multiple Resource Inventory’, Journal of Forestry, vol. 103, no. 6, pp. 286-292. Robinson, G J 1994, ‘The Accuracy of Digital Elevation Models Derived From Digitized Contour Data’, Photogrammetric Record, vol. 14, no. 83, pp. 805-814. Schaffrin, B, & Felus, Y A 2008, ‘On The Multivariate Total Least-Squares Approach To Empirical Coordinate Transformations Three algorithms’, Journal of Geodesy, vol. 82, no. 6, pp. 373-383. Teunissen, P J, Odijk, D&Zhang, B 2010, ‘PPP-RTK: Results of CORS Network-Based PPP with Integer Ambiguity Resolution’, Journal of Aeronautics, Astronautics and Aviation, Series A, vol. 42, no. 4, pp. 223-230. Read More